Spray Synthesis of CuFeO2 Photocathodes and In-Operando Assessment of Charge Carrier Recombination

Semiconducting delafossite-phase CuFeO2 is a promising photocathode material for solar-driven hydrogen production given its suitable energy levels and established robustness for photoelectrochemical water reduction. Nevertheless, reported methods for preparing CuFeO2 thin films are cumbersome, and an explanation for the observed poor performance of this material remains under debate. Herein, a facile and scalable deposition method based on aerosol-assisted chemical vapor deposition of CuFeO2 photocathodes is reported. Optimization of deposition conditions reveals that larger grain size is beneficial for photoelectrochemical operation. Extensive photo-electrochemical testing including illumination and potential modulation spectroscopy of these photocathodes under non-sacrificial operation conditions indicates solar photocurrent densities up to 2.5 mA cm(-2). The bulk charge separation efficiency and the interfacial charge injection efficiency at +0.4 V vs RHE are estimated to be 11% and 0.8%, respectively. This establishes that, while bulk photogenerated charge carrier recombination in CuFeO2 remains an important loss, the performance bottleneck of CuFeO2 for H-2 production is clearly due to surface recombination.

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

With growing concerns on future energy supplies, solar energy appears as an energy source whose potential remains to be tapped at a large scale. In the last two decades, dye-sensitized solar cells (DSCs) have been considered as a competitive means to convert solar energy into electricity thanks to their low manufacturing and environmental costs, simple fabrication and high efficiency. Indeed, record efficiencies as high as 13% have been obtained for liquid DSCs and recently, solid-state DSCs using solid hole transport materials as well as perovskite materials as sensitizers have showed efficiencies above 17%. From a physical point of view, charge carrier generation in a DSC takes place in a photoactive light harvester chemisorbed on a mesoscopic semiconductor oxide. Due to the excellent electronic overlap, the exciton separation occurs by the injection of the excited electrons and holes from the dye into the conduction band of the semiconductor and redox electrolyte, respectively. A high surface area mesoscopic photoanode is necessary to ensure a high-density bulk heterojunction and efficient light harvesting in the visible part of the solar spectrum due to effective dye loading. This implies that the DSC has a large, heterogeneous interface through which photogenerated electrons may be intercepted as a result of the slow electron transport. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a long path to the transparent conductive electrode. It is however hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film. The work conducted during this thesis aimed at developing new architectures for the electrodes of DSCs using notably atomic layer deposition (ALD) and inverse opal host-guest structures with different semiconducting metal oxides. First, using a self-assembled polymeric template and ALD, we fabricated a porous ZnO inverse opal backbone. Advantages are numerous: a high electronic mobility of ZnO, a precise control of the metal oxide deposition using ALD and in situ growth of ZnO nanowires at low temperatures. Besides increasing the surface area available for photoactive dye molecules, single-crystal nanowires offer a defect-free path for photoinjected electrons. However, because of the relative instability of ZnO in acidic environments, a TiO2 overlayer was deposited by ALD. This proved to increase the recombination resistance of photogenerated electrons and increased their lifetime in the porous film. This host-guest approach was further transposed to three related fields in photo-electrochemistry using different ALD precursors. For water splitting, a niobium-doped tin oxide inverse opal backbone was constructed, which served as a host for hematite deposited by atmospheric pressure chemical vapour deposition or ALD. In the field of plasmonics, a TiO2 inverse opal host was made to act as a scaffold for silver nanoparticles. For solid-state DSCs, we strived to achieve an ultra-thin ZnO inverse opal, which acted as a host for perovskite crystals. Finally, we studied the effect of very thin TiO2 and ZnO overlayers deposited by ALD on top of mesoporous ZnO and TiO2, respectively. We were able to accurately monitor the thickness of overlayers and study the effects on photovoltaic parameters such as transport and recombination rates of photogenerated electrons, photocurrent, photovoltage, fill factor and efficiency.

EPFL2014

Solar-driven reduction of CO₂

Marcel Roland Schreier

Rapidly increasing levels of atmospheric carbon dioxide and their damaging impact on the global climate system raise doubts about the sustainability of the fossil resource based energy system. Meanwhile, raising living standards and increasing global population lead to an ever growing need for energy. Renewable energy sources are believed to present a solution to these problems with the sheer abundance of solar energy showing particular promise to fulfill the world's energy needs. However, for large scale application of solar energy to be possible, the problem of its storage has to be addressed. The insufficient flexibility of present-day storage technologies has led to the quest for producing solar fuels, centering on hydrogen as a fuel in a prospective hydrogen economy. Nevertheless, the gaseous state, low volumetric energy density and explosive nature of hydrogen makes it a challenging fuel for practical applications. Using solar energy to produce carbon-based liquid fuels solves these challenges, closes the anthropogenic carbon cycle and allows for the continued utilization of existing infrastructures. A promising method for the production of such fuels consists in the photoelectrochemical and electrochemical conversion of carbon dioxide. In this thesis, both methods are investigated using molecular homogeneous catalysts and heterogeneous systems. The photoelectrochemical reduction of carbon dioxide on TiO2-protected Cu2O photocathodes was investigated using a rhenium bipyridyl catalyst in solution. Important charge transport limitations were encountered, which could be overcome by the addition of protic additives to the electrolyte. Improving on this result, the molecular catalyst was covalently immobilized on the TiO2 surface of the photocathode by modifying the bipyridyl ligand with a phosphonate binding group. A nanostructure of TiO2 was needed to support sufficient catalyst to sustain the photocurrent generated by the Cu2O photoelectrode. The complete device showed photocurrents exceeding 2.5 mA cm-2 and large faradaic efficiency for the production of CO. Moving toward heterogeneous catalysis, the promotion of the CO2 to CO conversion reaction on silver surfaces by imidazolium cations was investigated. Replacing the imidazolium C2 proton with a phenyl substituent led to an enhancement of the co-catalytic effect. Replacing the C4 and C5 protons with methyl groups, however, suppressed the catalysis-promoting effect of the imidazolium salt for different C2 substituents and led to new insights into the role of imidazolium. The unassisted solar-driven splitting of CO2 into CO and O2 was demonstrated using water as electron source. This was achieved by the use of a porous gold cathode and an IrO2 anode, driven by three methylammonium lead iodide perovskite solar cells in series. Extended operation over 18 h was shown, achieving a solar to CO efficiency exceeding 6.5 %. Atomic layer deposition (ALD) modification of CuO nanowire cathodes with SnO2 was investigated, leading to striking impacts on the catalytic selectivity of this system. In an aqueous electrolyte, bare CuO led to the production of a wide spectrum of products, which was modified to the production of CO with high selectivity upon ALD modification. By exploiting the oxygen evolving activity of SnO2-coated CuO, a low cost bifunctional system was constructed, achieving sustained solar-driven production of CO with up to 13.4% efficiency.

EPFL2017

Solar photoelectrolysis of water with translucent nano-structured hematite photoanodes

Ilkay Cesar

In the context of the diminishing fossil energy resources and global warming, much research is focused on sustainable energy sources. The research that is presented in this thesis falls in this category as it contributes to the development of a device that stores solar energy into a chemical fuel. This device is based on the photoelectrolysis of water in a tandem-cell that produces hydrogen and oxygen under illumination of sunlight. This can be achieved by connecting a larger band gap semiconductor photoanode for oxygen evolution (such as WO3 with Eg = 2.6 eV or Fe2O3 with Eg = 2.0 eV) in series with two dye sensitized solar cells and a hydrogen evolving cathode. Red and green light transmitted by the photoanode is absorbed by the dye, so that a large part of the solar spectrum can be utilized. Fe2O3 absorbs a larger part of the visible solar spectrum as compared to WO3, which results in a 3 times higher theoretical conversion efficiency for the tandem-cell. However, the small hole diffusion length in Fe2O3 of 2 to 20nm is seen as an important reason for the lower practical conversion efficiencies reported in literature. The aim of this thesis project is to improve the photooxidation activity of iron oxide photoanodes in order to improve the practical conversion efficiency of the tandem-cell. For this purpose we have prepared thin films of nano-structured iron oxide with various deposition methods and characterized their performance on the photooxidation of water. The photoanodes are measured in aqueous solution of 1 M NaOH (pH=13.6) under illumination of simulated sunlight (AM 1.5 global of 1000 W/m2). We report the photocurrent at the reversible water oxidation potential of 1.23 V vs. RHE. The first chapter briefly discusses the potential of hydrogen as an energy carrier in a global hydrogen economy and places the tandem-cell in this context. In chapter two the current status of photoelectrolysis of water with iron oxide is briefly reviewed. The experimental set-up for the measurement of the photocurrent under simulated sun light and for the acquisition of photocurrent action spectra are described. This chapter also deals with the spectral mismatch error that is associated with the solar simulator calibration procedure. Chapter three introduces a new solution strategy for efficient photoanodes based on a nanocomposite structure with a hematite film thickness that is commensurate to the hole diffusion length alleviating the problem of poor charge transport. For this purpose nano-porous films of doped and undoped tinoxide and arrays of perpendicularly oriented nanorods of the same material are conformally coated with a thin film of hematite (2-20nm). These electrodes are characterized by SEM, TEM, XEDS elemental analysis, Raman spectroscopy in addition to the photoelectrochemical response. Although, evidence is presented for the successful preparation of the nano-composite structures with a sufficient optical density, the photocurrents obtained from these electrodes are very low. In chapter four we report on thin silicon-doped nanocrystalline α-Fe2O3 films (thickness of 250-500nm) that have been deposited on F-doped SnO2 substrates by ultrasonic spray pyrolysis (USP) and chemical vapor deposition at atmospheric pressure (APCVD). The photoanodes prepared by USP and APCVD gave a photocurrent of 1.17 and 1.45 mA/cm2 respectively. The morphology of the α-Fe2O3 was strongly influenced by the silicon doping, decreasing the feature size of the mesoscopic film. The silicon-doped α-Fe2O3 nano-leaflets show a preferred orientation with the (001) basal plane normal to the substrate. Chapter five treats the preparation and characterization of hematite photoanodes prepared by an improved APCVD setup. Under illumination water is oxidized at the Fe2O3 electrode with higher efficiency (IPCE = 42% at 370 nm and 2.2 mA/cm2) than at the best reported single crystalline Fe2O3 electrodes. This unprecedented efficiency is in part attributed to the dendritic nanostructure which minimizes the distance photo generated holes have to diffuse to reach the Fe2O3/electrolyte interface while still allowing efficient light absorption. Part of the gain in efficiency is obtained by depositing a thin insulating SiO2 interfacial layer between the SnO2 substrate and the Fe2O3 film, and a catalytic cobalt monolayer on the Fe2O3 surface. A mechanistic model for water photooxidation is presented, involving stepwise accumulation of four holes by two vicinal iron or cobalt surface sites. In chapter six we present a detailed investigation of the dependency of the photocurrent response on film thickness and feature size of doped and undoped hematite photoanodes prepared by the APCVD setup discussed in chapter five, in absence of a redox catalyst. Evidence is presented of an unusual high donor density of 1020 cm-3 in the silicon doped photoanodes which would allow a space charge layer to be formed inside the nano-sized features of the polycrystalline film. This suggests that electric field induced charge separation could be an important reason for the photocurrent improvement reported in chapter five. In addition, a strong correlation between feature size and the intensity of a Raman band at 660cm-1 is presented and discussed in the context of disorder in the crystal structure. Chapter seven presents a study of the influence that the deposition parameters have on the photoresponse, morphology, and crystal structure of films prepared by ultrasonic spray pyrolysis. Films are characterized by SEM, XEDS elemental analysis, TEM, XRD, Mössbauer and Raman spectroscopy. A strong correlation is found between the formation of β-Fe2O3 (Bixbyite) and low photocurrents. Silicon doped films enable the electron transport across a film thickness of 1300nm. The absorbed-photon-to-current-conversion-efficiency (APCE) increases with film thickness below 150nm, which indicates an increased recombination near the substrate interface. This could be an important reason for the low photoresponse of nanocomposite electrodes studied in chapter three. In chapter eight we draw some general conclusions with regards to the future development of these photoanodes.

EPFL2007